TYRES AND FUEL CONSUMPTION
Tyres are The control of a vehicle. They are the means by which a vehicle is able steer, stop, and GO. This is commonly referred to as traction. Traction is a result of many factors. Among the many factors that effect traction, the most important are the weight applied to the tyre and road, the condition of the road, the condition of the tyre, and the amount of surface contact between the tyre and the road. For a given size of tyre, any increase or decrease in weight will result in a corresponding change in traction. As a road becomes more or less slippery, so does traction decrease or increase. Surface contact between the tyre and the road is a factor of tire size and shape as well as tyre inflation. Tyres must not be either too large or too small for the vehicle; follow the manufacturer's recommendation for tyre size. For a tire to have its maximum designed contact with the road it must not be either over or under inflated. An overinflated tire reduces the tyre and road surface contact patch and causes the tyre to wear excessively in the center area of the tread. See Figure 1 for a tyre that has been run overinflated.
Figure 1
An underinflated tyre also reduces the tyre to road contact patch area and will wear the tyre excessively on the edges of the tyre. Additionally, underinflated tyres tend to squirm, that is they wiggle from side to side instead of tracking in a straight line. It is the loss of contact patch and squirming that causes a loss in fuel economy. See Figure 2 for the typical tread wear due to underinflation.
Figure 2
As tyres roll a certain amount of drag occurs. Due to alignment geometry, this drag tends to make the front tyres of a rear wheel drive vehicle move in an outward fashion as the vehicle moves forward. A front wheel drive vehicle's tyres tend to move in an inward fashion as the vehicle moves forward. As the tyres move in this outward or inward fashion, tyres experience greater amounts of drag and resistance increased tyre wear rates and decreased fuel economy results. To counteract these effects, alignment geometry is statically adjusted toe-in or toe-out so that the vehicles tyres roll in a straight, non-scuffing, and least resistant manner. Proper toe setting of the front wheels is very important to tyre life as well as fuel economy. See Figure 3 for a tyre which has been subjected to an improper toe setting. Vehicles equipped with independent rear suspensions require periodic alignment corrections; solid rear axles rarely lose their alignments.
Figure 3
Out-of-balance tyres may also affect MPG. If a tyre is unbalanced, the unbalanced dynamic forces causes the tyre to run eccentrically. This eccentric movement causes reduced tire traction. The loss of traction results in a power loss, thus MPG is reduced. Additionally, unbalanced tyres wear out faster. See Figure 4 for the effect on a tire that has been improperly balanced.
Figure 4
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